1. Technical Field of the Invention
This invention relates generally to oscillator circuits, and in particular, to crystal oscillator circuits.
2. Description of Related Art
Local oscillator signals used in cellular telecommunications applications must be both tunable and highly stable. A tunable frequency can be easily produced using an LC type oscillation circuit. However, LC oscillators typically do not have sufficient frequency stability for cellular applications. Therefore, crystal oscillators are often used to provide the necessary frequency stability. Crystals, such as quartz, have an extremely high Q, which leads to oscillators with very stable frequency values.
Typically, quartz crystals are cut and mounted to vibrate best at a desired resonant frequency or an overtone (multiple) of the desired resonant frequency. When the crystal is vibrating, the crystal can be modeled as an RLC circuit that produces a rapidly changing reactance with frequency, with the RLC circuit providing positive feedback and gain at the resonant frequency, leading to sustained oscillations. Although the crystal is designed to oscillate at its resonant frequency, in order to provide tunability, a circuit can be coupled to the crystal oscillator to “pull” the frequency of the crystal oscillator to a desired value.
The simplest form of a voltage-controlled crystal oscillator is a single-ended oscillator circuit, in which single-ended signals are used to initiate and maintain the crystal oscillations. However, single-ended designs often suffer from excessive noise due to interference from the substrate of the oscillator circuit and from the bonding wires coupled between the oscillator circuit and crystal. As a result, differential oscillator circuits are becoming more widely used in cellular applications due to their ability to suppress some of the noise.
Differential crystal oscillator circuits typically utilize a current source to provide the bias current to drive the differential oscillator circuit. However, in traditional differential designs, the flicker noise induced by the current source contributes significantly to the overall circuit phase noise (PN). Thus, it has been difficult to meet the stringent PN requirements (e.g., −150 dBc/Hz at 10 kHz offset) in cellular applications with traditional differential crystal oscillator designs.